The present invention generally relates to a water-free organic electrolyte for galvanic cells with a negative light-metal electrode, particularly electrolytes which contain at least one organic solvent with a CH bond activated by an --O--,.dbd.CO or .dbd.N-neighboring group.
Solvents of this type are commonly found as an electrolyte component, especially in high-energy lithium cells. They are selected, on the one hand, because of their complete inertness to the highly reactive electrode material, and on the other hand, because of their good solvation capacity for inorganic salts.
The ether group has been found to satisfy these requirements to a high degree. Ethers are also designated as aprotic solvents because they do not yield protons, and therefore do not react with alkali metals. Instead, they behave as Lewis bases. The following are examples of ethers used in lithium cells: the aliphatic monoethers such as dimethyl ether, diethyl ether, diisopropyl ether and n-butyl ether, the aliphatic polyethers such as diethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether and tetraethylene glycol dimethyl ether, and finally, cyclic ethers such as tetrahydrofuran, 1,4-dioxane and tetrahydropyran.
Other electrolyte solvents which may be used include butyrolactone and acetonitrile, for example.
Mixtures of several solvents are usually used in actual practice, especially when there is a need to compromise between, for example, the solubility of the conductive salt in the electrolyte solvent and the vapor pressure behavior, or when it is necessary to have a favorable viscosity (for which purpose, a PC/LiClO.sub.4 electrolyte is diluted with DME, for example).
However, a broken CH bond in such organic liquids makes them receptive to the acceptance of oxygen. The ethers show an especially marked tendency to oxidize to nonvolatile peroxide. The resulting potential for explosion has proven to be a significant hindrance in handling ether-containing high energy cells.
Ether peroxides usually form from hydroperoxide, which initially arises from the cleavage of a CH bond next to the oxygen bridge of the ether molecule (which then incorporates O.sub.2), and which subsequently decomposes into an unstable alkyl peroxide radical and an alcohol. The alkyl peroxide radicals then combine into corresponding peroxide polymers via a radical chain mechanism. Therefore, both hydroperoxide monomers and alkylidene peroxide polymers can be isolated from peroxide-containing ethers.
Related experience with exploded lithium cells (both in test and in actual use) has demonstrated that oxidation processes of this type also occur in the cells, despite hermetic sealing from the outside air. Very small amounts of oxygen can be formed in the cell by decomposition of oxygen-containing compounds such as LiClO.sub.4, CrO.sub.x and MnO.sub.2, or by electrolysis of residual H.sub.2 O in the not completely water-free solvent. It is also possible for the cell to be contaminated by peroxide formed in the purification of the ether by distillation. All that is then needed to trigger an explosion is an unintentional energy source, which can be provided, for example, by a short circuit, with subsequent overheating of the cell, or by the action of light.
Previous measures attempting to protect against explosion have been directed (without specific reference to the present electrochemical system) either to completely avoiding the threat of explosion of the cell by an early interruption of current (e.g., by means of a fusible conductor element), or to moderate the power of the explosion, and thus its destructiveness, by mechanical measures. The latter can be achieved, for example, by valve designs in which a deformable ball is mounted in a matching opening in the area of the cell cover to normally provide a seal, but so that when a maximum permissible internal pressure is exceeded, the ball deforms irreversibly and creates an opening.